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Saturday 15 August 2020

The West Angle sediment sequence: Morey's paper

Generalised sequence of Pleistocene deposits at West Angle

I thought I should draw attention to the fact that the article by CR Morey on the West Angle sediment sequence has now been placed online by the Ussher Society:

The main interest of this work lies in the confirmation by Morey that the silt and clay series at West Angle is not a result of a big marine transgression, but is a series of muds and clays laid down in a dune slack situation, probably behind a coastal dune barrier.  The evidence of freshwater organic materials is quite convincing, and most authors are now agreed that the organic sediments (including a peat bed which has been eroded away) probably date from the last interglacial.

However, I part company with Morey when he interprets the overlying reddish diamicton as a periglacial solifluction deposit. This is a weird and unnecessary interpretation of the prominent deposit which occupies the northern part of the cliff exposure:

Exposure of the dark red Irish Sea till in the section in West Angle Bay.  The colouration is down to the inclusion of much debris from overridden ORS rocks.

As I wrote in 2017, Morey's illustration of the sequence is so generalised and inaccurate as to be effectively useless.  He does not accept that there is a genuine raised beach deposit near the base of the succession at West Angle, and nor does he accept that his "reddish brown" layer 4(i) is a till layer, in spite of recording within it clasts of ORS marls and sandstones, dolerites and rhyolites. He prefers to follow Bowen (1974) in interpreting this as a periglacial head or slope deposit accumulated under permafrost conditions. He does not explain where all this erratic material might have come from. Nor does he seem to have examined the fresh till that is exposed in the northern coves of West Angle Bay; such an examination might well have convinced him that Late Devensian ice did indeed reach this locality and must have affected the sedimentas at the head of the main bay.

Since 2017 I have made abundant observations on the west and south coasts of Pembrokeshire which have confirmed that almost all of this coastline was affected by overriding ice at the time of the LGM.    Morey's interpretation of the reddish diamicton was no doubt influenced by the widespread assumption (in 1997) that the Late Devensian Irish Sea Glacier dod not extend this far south -- and that therefore everything that looked like a till was probably something else..........  That assumption has now been comprehensively dumped.

This is the crucial part of the West Angle sequence -- a dramatic erosional contact between the interglacial silt and clay series (to the right) and the dark red Late Devensian till (to the left)

So we can suggest that the stratigraphic and age relationships of the West Angle deposits are as follows:

10. Made ground and soil -- modern
9. Dark red stratified horizon -- late glacial (Devensian / Holocene transition
8. Dark red diamicton (non-stratified) -- Late Devensian glaciation (LGM)
7. Orange silt and clay series -- Ipswichian interglacial dune slack environment (freshwater)
6. Grey silt and clay series -- ditto
5. Peat and peaty silt -- ditto
4. Stony grey silts -- up to 1.5 m thick -- ditto (includes some slope breccia material?)
3. Ferruginous bedded sands and gravels -- up to 1.5 m thick -- Ipswichian shoreline deposits?
2. Rounded pebbles / beach shingle in a sandy and gravelly matrix -- up to 1.8 m thick -- Ipswichian raised beach
1. Sand layer -- more than 1 m thick -- Ipswichian sandy beach
Base -- pre-Ipswichian till?  Claimed by some -- not seen by me!




Morey, C.R. 1997. A re-examination of the valley-fill deposits at West Angle Bay, Pembrokeshire.Proceedings of the Ussher Society, 9, 164-167.


The cliffs at West Angle Bay in Pembrokeshire [SM 853 031] display till, apparently of pre-Devensian age, overlain by a sequence of Late Pleistocene deposits. This section has been the subject of conflicting interpretations and has remained enigmatic since it was first described by Cantrill for the British Geological Survey in 1916. A re-examination was prompted by the discovery of new macrofossil material and the fact that, although part of the section has been described as "marine" or "estuarine" in character by various authors, there is no published record of any systematic micropalaeontological work to support such an interpretation.

The picture that has emerged from the current work is that of a valley excavated in bedrock during the Early Pleistocene and subsequently filled by till during a glaciation which transported erratics of igneous material and sediments of Mesozoic and Westphalian age from the north and west. During a subsequent interglacial, drainage was impeded by the build-up of a dune belt at the coast. Alluviation took place in a slack behind the dune belt and it is some of this material that is now exposed in the coast section.

Subsequent cold cycles were marked by gully erosion and by two phases of solifluction. Deposits from the early phase are distinguished by the fact that they contain re-distributed erratic material. There is also evidence for landslipping or subsidence involving partially frozen ground, perhaps following the erosion of a coastal dune belt.

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